Turning is the machine shop operation that generates an exterior surface, circular in its transverse cross-section on workpieces that rotate about an axis. Turning is usually performed with a single point tool.
The corresponding operation that generates an interior surface is called "boring"; that is, boring constitues internal turning. Either the workpiece or the single-point cutting tool may rotate about an axis to enlarge in size a hole that was originated previously by coring, flame-cutting, drilling, or other methods. The single-point cutting tool can also be used to thread either interior or exterior surfaces on a workpiece. Conventionally, lathes and boring or turning mills employ single-point cutting tools.
One of the most commonly employed single-point cutting tools consists of: a bar, normally round in its cross-section, one of whose ends can be mounted on a chuck or other suitable gripping device (the slave end), and whose other end (the free end) is shaped to accept a tip holder; a tip holder, constituting a device shaped for mounting on the free end of the bar, and having provisions for accepting and mounting a cutting tip; and the cutting tip.
The cutting tips commercially available today have been largely standardized in design by their various manufacturers for convenience of interchangeability. Thus, most cutting tips are flat, triangular in shape, and include a hole through their center. A cam screw is passed through the tip holder, and an eccentric extension pin passes through the hole in the cutting tip and the cutting tip is pressed into contact with a recess surface of the tip holder by rotation of the screw with shifting of the eccentric acting to fasten and fix the cutting tip firmly with respect to the tip holder and in contact with the tip holder surface.
On the other hand, the bar and tip holder combination is available in a great variety of designs. For example, the bar and tip holder may be respectively beveled, lipped, notched or otherwise designed so that a portion of their surfaces mate closely.
However, all presently known designs include certain common features. Both the bar and tip holder have screw holes which become aligned when the tip holder is in its proper position on the bar; securing screws are then passed through the aligned holes, and the securing screws are tightened to fasten and fix the tip holder firmly on the bar. Also, all such designs have at least one of the securing screws (and, consequently, its hole) oriented radially along the bar transverse cross-section.
The main disadvantage of having radially oriented securing screws is that the resolution of the forces acting upon the tool is unsatisfactory. The tangential force acting against the cutting tip during operation is transmitted as stress through the tip holder, to the securing screws as stain and shear forces, and to the bar as torque. Whereas, the tip holder is usually of a material hard enough to withstand the stress, the strain on the securing screws can elongate the securing screws or deform their threads (thus deforming the thread of its hole through the tip holder and/or--more likely--the thread of its hole through the bar). The shear force on a securing screw can eventually break the screw or, even worse, cause the screw hole to widen inside the bar. If the bar is thus weakened, it will likely succumb to the torque and deform or twist. If larger-diameter securing screws are employed in order to increase their resistance to strain and shear, their correspondingly larger-diameter screw holes reduce the cross-section of the bar and the bar becomes more susceptible to deformation or twisting by the torque.
Another disadvantage of the conventional design is that only one end of the bar can serve as its free end. Radially oriented screws require a very tight mating of the surfaces of the bar and the tip holder in order to align the securing screw holes. Since the slave end of the bar is gripped by pressure in the jaws of, for instance, a chuck, there is a great likelihood of damage to the surface on the slave end of the bar, and consequently there is a great likelihood that the securing screw holes may become misaligned. Thus, it is impractical to design both ends of the bar to serve as free ends capable of accepting a tip holder.
Attempts have been made to manufacture single-point cutting tools which eliminate the tip holder by incorporating its function in the bar. In other words, the bar itself has provisions for accepting a cutting tip. While this design avoids the problems created by radially oriented securing screws, it is impractical because it requires the bar then to be made of a material hard enough to withstand the concentrated stress acting on the surface supporting the cutting tip.
It is, therefore, a primary object of the present invention to provide an improved single-point cutting tool which eliminates the transmission of forces generated during use through the securing screw and to eliminate the strain and the shear force acting on the screw.
It is a further object of the present invention to provide an improved single-point cutting tool in which the screw hole for the tip holder securing screw is provided with respect to the bar so as to minimize the capability of the bar to withstand applied torque during cutting.